1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly to an air/fuel ratio control system using a linear type oxygen concentration sensor and having a function for preventing the so-called blackening phenomenon of the oxygen concentration sensor.
2. Description of Background Information
Air/fuel ratio feedback control systems for an internal combustion engine are well known, in which the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and an air/fuel ratio of mixture to be supplied to the engine is feedback controlled in response to a result of the detection of the oxygen concentration so as to purify the exhaust gas and improve the fuel economy.
As an example of this air/fuel ratio control system, a duty ratio control type secondary air supply system has been proposed which is provided with an open/close valve disposed in an air intake side secondary air supply passage leading to the carburetor, downstream of the throttle valve. The open/close valve is controlled in such a manner that a duty ratio between the opening and closing of the open/close valve is controlled in response to an output signal level of the oxygen concentration sensor.
In such an air/fuel ratio control system, a detection operation as to whether the air/fuel ratio of a mixture is lean or rich with respect to a target air/fuel ratio is performed by using the output signal level of the oxygen concentration sensor. When a result of the detection indicates that the air/fuel ratio is lean, a base valve open duration is decreased by a predetermined amount in each duty cycle of valve operation. On the other hand, if the result of detection indicates that the air/fuel ratio of the mixture is rich, the base valve open duration in each duty cycle is increased by a predetermined amount. The thus produced calculation value of an integration control operation is used as an output valve open time duration. The open/close valve is opened for this output valve open time duration.
In another case, a correction value for the integration control operation is decreased by a predetermined value when the result of detection indicates that the air/fuel ratio is lean. When, on the other hand, the result of detection indicates that the air/fuel ratio is rich, the correction value is increased by a predetermined amount. The correction value is then added to a base valve open time duration in each duty cycle. The thus obtained calculation value is used as the output valve open time duration for which the open/close valve is opened in each duty cycle of valve operation.
As an example of an oxygen concentration sensor for use in air/fuel ratio control systems of the above mentioned type, Japanese patent application Laid Open No. 58-153155 discloses an oxygen concentration sensor having an output signal whose level is proportional to the oxygen concentration in a measuring gas (whose oxygen concentration is to be measured) when the air/fuel ratio of the mixture supplied to the engine is larger than a stoichiometric air/fuel ratio. This oxygen concentration sensor includes a pair of flat oxygen-ion conductive solid electrolyte members. The oxygen-ion conductive solid electrolyte members are placed in the atmosphere of an exhaust gas of the engine. Electrodes are respectively provided on the front and back surfaces of both of the solid electrolyte members. In other words, each pair of electrodes sandwich each solid electrolyte member. These two solid electrolyte members each having a pair of electrodes are arranged in face to face relation with each other to form a gap portion between them.
With this arrangement, one of the solid electrolyte members operates as an oxygen pump element and the other one of the solid electrolyte members operates as a sensor cell element for measuring an oxygen concentration ratio. In the atmosphere of the test gas, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrode facing the gap portion is used as a negative electrode. By the supply of this current, the oxygen component of the gas within the gap portion is ionized on the surface of the negative electrode of the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface of the positive electrode in the form of the oxygen gas.
While this movement of oxygen ions is taking place, an electric potential is generated across the electrodes of the sensor cell element because the oxygen concentration of the gas in the gap portion differs from the oxygen concentration of the gas outside the electrodes of the sensor cell element. If the magnitude of the electric current supplied to the oxygen pump element, that is "the pump current", is constant, the electric potential generated across the sensor cell element becomes proportional to the oxygen concentration difference, i.e, the oxygen concentration in the exhaust gas.
By using this electric potential developed across the electrodes of the sensor cell element, the detection is performed as to whether the air/fuel ratio of the mixture supplied to the engine is rich or lean with respect to the target air/fuel ratio. In addition, the target value of air/fuel ratio is determined to be larger than the stoichiometric air/fuel ratio. Further, if the magnitude of the pump current to be supplied to the pump element is varied so that the electric potential across the electrodes of the sensor cell element becomes constant, the magnitude of the pump current becomes substantially proportional to the oxygen concentration in the exhaust gas under a condition of room temperature. Therefore, the air/fuel ratio can be also detected by means of the magnitude of the pump current.
In this type of oxygen concentration sensing devices, if an excessive current is supplied to the oxygen pump element, it causes the so called blackening phenomenon by which the oxygen ions are removed from the solid electrolyte members. For instance, when zirconium dioxide (ZrO.sub.2) is utilized as the solid electrolyte, the oxygen ions O.sub.2 are taken from the zirconium dioxide (ZrO.sub.2) so that zirconium (Zr) is separated out. As a result of this blackening phenomenon, a deterioration of the oxygen pump element takes place rapidly, to cause a debasement of the operation of the oxygen concentration sensor as a whole.
FIG. 1 shows curves indicating current I.sub.p to the oxygen pump element versus oxygen concentration relations and a boundary line of the occurrence of the blackening phenomenon. As illustrated, the magnitude of the current I.sub.p varies in proportion to the oxygen concentration, and the rate of variation is different for several different values of the voltage Vs developing across the electrodes of the sensor cell element. In other words, the voltage Vs is a parameter which determines the relation between the magnitude of the current I.sub.p and the oxygen concentration. As illustrated in this figure, the boundary line of the occurrence of the blackening phenomenon is expressed, as in the case of the magnitude of the current I.sub.p, as a first-degree function of the oxygen concentration value. Therefore, for preventing the blackening phenomenon, it is necesssary that the magnitude of the supply current to the oxygen pump element is limited to be smaller than values in the region of the blackening phenomenon.
In air/fuel ratio control systems using this type of oxygen concentration sensing device, the calculated output valve open time may exceed the above mentioned upper limit of the valve open period due to a deviation of a basic air/fuel ratio set value of the carburetor, or when the auxiliary power supply system such as a power valve is in operation, or during an engine operation after a hot start of the engine. If the valve open period exceeds the upper limit of the valve open period, the amount of air flowing through the open/close valve will be saturated. Therefore, under such a condition, the output valve open period is set to be equal to the above mentioned upper limit value of the valve open period. Therefore, the air/fuel ratio of the mixture supplied to the engine is not made lean enough. Under such a condition, the air/fuel ratio is at around a stoichiometric value or a value slightly richer than the stoichiometric value.
However, there is a relation that the richer the air/fuel ratio becomes, or in other words, the smaller the oxygen concentration becomes, the lower becomes the boundary level of the occurrence of the blackening phenomenon. Therefore, under such a condition of the air/fuel ratio, the value of the current supplied to the oxygen pump element is above a critical value of the occurrence of the blackening phenomenon. Thus in conventional arrangements, there has been a chance of occurrence of the blackening phenomenon.